1. Technical Field
The invention relates to spacer grids for a nuclear fuel assembly, of the type comprising two sets of intersecting plates for defining fuel rod reception cells. Such grids are used for retaining the rods at the nodal points of a regular array, square or triangular. To retain the grids on the rods while allowing the latter to slide upon differential expansion of the rods with respect to each other or with respect to tie rods or guide thimbles on which the grids are fixed, the plates are generally provided with springs which each apply the rods against a fixed boss or dimple of the plate opposite the one carrying the spring, or against another spring.
When the fuel rods are inserted directly in the grids of an assembly, it is necessary to exert thereon an insertion force which is greater than the friction force exerted by the springs. Caution must be taken not to exert an excessive force, and this increases the time duration for manufacturing the assembly. The friction of the rods on the springs and the bosses may cause scratching unfavorable to the resistance of the sheaths of the rods under irradiation.
2. Prior Art
Attempts have already been made to solve the problem raised by the friction of the fuel rods on the springs. In particular, it has been suggested first to insert a bundle of dummy rods into the grid for forcing back the springs, then to retain the springs in retracted condition with a comb formed of wires of small diameter inserted in the grid before removing the dummy rods and inserting the actual fuel rods (European No. 0 196 609). This solution is complex and it is difficult to correctly insert the set of wires forming the comb in a position in which the wires retain all springs, without jamming.